Preparing Gases in the Lab · GasLab

Oxygen from heating potassium permanganate

Potassium permanganate is a purple-black crystal that gives off oxygen when heated. Oxygen does not dissolve easily in water, so it can be collected over water.

Remember the seven steps: check, load, fix, light, collect, remove, extinguish. Work through them in order — or get one wrong on purpose and see what happens.

Oxygen from decomposing hydrogen peroxide

Hydrogen peroxide solution decomposes very slowly at room temperature; with manganese dioxide added it releases oxygen quickly, while the manganese dioxide keeps its mass and chemical properties — it is a catalyst.

This apparatus needs no heating. A separating funnel controls how fast the liquid drips in, and so how fast the reaction goes. Let some drip in without manganese dioxide first, then add it and compare how fast it bubbles.

Preparing carbon dioxide

In the lab, carbon dioxide is made by reacting marble (or limestone) with dilute hydrochloric acid; no heating is needed.

Carbon dioxide dissolves in water and is denser than air, so it is collected by upward displacement of air; to test whether the jar is full, hold a burning splint at the mouth. Test it with clear limewater.

On the right, try dilute sulfuric acid, concentrated hydrochloric acid or marble powder instead, and think about why the lab does not use them.

Properties of oxygen and carbon dioxide

Full jars of oxygen and carbon dioxide are ready on the bench.

Oxygen supports combustion: a glowing splint relights, and iron wire burns fiercely in oxygen, throwing out sparks.

Carbon dioxide does not burn or support combustion, is denser than air, and reacts with water to form carbonic acid.

In-class testing

Look back over the four experiments and answer the multiple-choice questions below.